New Insights Into What Might Have Smashed Uranus Over Onto its Side

Uranus

The gas/ice giant Uranus has long been a source of mystery to astronomers. In addition to presenting some thermal anomalies and a magnetic field that is off-center, the planet is also unique in that it is the only one in the Solar System to rotate on its side. With an axial tilt of 98°, the planet experiences radical seasons and a day-night cycle at the poles where a single day and night last 42 years each.

Thanks to a new study led by researchers from Durham University, the reason for these mysteries may finally have been found. With the help of NASA researchers and multiple scientific organizations, the team conducted simulations that indicated how Uranus may have suffered a massive impact in its past. Not only would this account for the planet’s extreme tilt and magnetic field, it would also explain why the planet’s outer atmosphere is so cold.

The study, “Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion“, recently appeared in The Astrophysical Journal. The study was led by Jacob Kegerreis, a PhD researcher from Durham University’s Institute for Computational Cosmology, and included members from the Bay Area Environmental Research (BAER) Institute, NASA’s Ames Research Center, the Los Alamos National Laboratory, Descartes Labs, the University of Washington and UC Santa Cruz.

Artist’s impression of the huge impact that is believed have formed the Moon roughly 4.5 billion years ago. Credit: Joe Tucciarone

For the sake of their study, which was funded by the Science and Technology Facilities Council, The Royal Society, NASA and the Los Alamos National Laboratory, the team ran the first high-resolution computer simulations of how massive collisions with Uranus would affect the planet’s evolution. As Kegerries explained in a recent Durham University press release:

“Uranus spins on its side, with its axis pointing almost at right angles to those of all the other planets in the solar system. This was almost certainly caused by a giant impact, but we know very little about how this actually happened and how else such a violent event affected the planet.”

To determine how a giant impact would affect Uranus, the team conducted a suite of smoothed particle hydrodynamics (SPH) simulations, which were also used in the past to model the giant impact that led to the formation of the Moon (aka. the Giant Impact Theory). All told, the team ran more than 50 different impact scenarios using a high-powered computer to see if it would recreate the conditions that shaped Uranus.

In the end, the simulations confirmed that Uranus’ tilted position was caused by a collision with a massive object (between two and three Earth masses) that took place roughly 4 billion years ago – i.e. during the formation of the Solar System. This was consistent with a previous study that indicated that an impact with a young proto-planet made of rock and ice could have been responsible for Uranus’ axial tilt.

Composite image of Uranus by Voyager 2 and two different observations made by Hubble — one for the ring and one for the auroras. Credit: NASA/ESA

“Our findings confirm that the most likely outcome was that the young Uranus was involved in a cataclysmic collision with an object twice the mass of Earth, if not larger, knocking it on to its side and setting in process the events that helped create the planet we see today,” said Kegerries.

In addition, the simulations answered a fundamental questions about Uranus that was raised in response to previous studies. Essentially, scientists have wondered how Uranus could retain its atmosphere after a violent collision, which would have theoretically blown off its out layers of hydrogen and helium gas. According to the team’s simulations, this was most likely because the impact struck a grazing a blow on Uranus.

This would have been enough to alter Uranus’ tilt, but was not strong enough to remove its outer atmosphere. In addition, their simulations indicated that the impact could have jettisoned rock and ice into orbit around the planet. This could then have coalesced to form the planet’s inner satellites and altered the rotation of any pre-existing moons already in orbit around Uranus.

Last, but not least, the simulations offered a possible explanation for how Uranus got its off-center magnetic field and its thermal anomalies. In short, the impact could have created molten ice and lopsided lumps of rock inside the planet (thus accounting for its magnetic field). It could have also created a thin shell of debris near the edge of the planet’s ice layer which would have trapped internal heat, which could explain why Uranus’ outer atmosphere experiences extremely cold temperatures of -216 °C (-357 °F).

This artist’s conception shows the Uranus-sized exoplanet Kepler-421b, which orbits an orange, type K star about 1,000 light-years from Earth. Credit: David A. Aguilar (CfA)

Beyond helping astronomers to understand Uranus, one of the least-understood planets in the Solar System, the study also has implications when it comes to the study of exoplanets. So far, most of the planets discovered in other star systems have been comparable in size and mass to Uranus. As such, the researchers hope their findings will shed light on these planet’s chemical compositions and explain how they evolved.

As Dr. Luis Teodoro – of the BAER Institute and NASA Ames Research Center – and one of the co-authors on the paper, said, “All the evidence points to giant impacts being frequent during planet formation, and with this kind of research we are now gaining more insight into their effect on potentially habitable exoplanets.”

In the coming years, additional missions are planned to study the outer Solar System and the giant planets. These studies will not only help astronomers understand how our Solar System evolved, they could also tell us what role gas giants play when it comes to habitability.

Further Reading: Durham University, The Astrophysical Journal

Why is Uranus on its Side?

It’s impossible to do an article about Uranus without opening up the back door to a spit storm of potty humour. I get it, there’s something just hilarious about talking about your, mine and everyone’s anus. And even if you use the more sanitized and sterile term urine-us, it’s still pretty dirty, in an unwashed New York stairwell kind of way. You’re in us? No.

This is a no-win solution. It’s a Kobayashi Maru scenario here. We’re all doomed.

Can we call a truce? I dare you commentators, to keep the YouTube comments as pure and clean as driven snow, so we can focus on the super interesting science. Think of the children.

Let’s set the stage, I’m going to let planetary astronomer Kevin Grazier give you the proper pronunciation to clear our minds and let us move forward with grace and civility.


Kevin Grazier:
Strictly speaking, it’s pronounced Youranous, is the  pronunciation.


As you probably know, Uranus… I mean Ouranus. No, I can’t do it, my brainwashing is too far along. Save yourselves!. Anyway, Uranus is the 7th planet from the Sun, and the 3rd largest planet in the Solar System. Jupiter and Saturn get all the spacecraft and Hubble space telescopes, but Uranus is an incredibly worthwhile target to visit.

Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA
Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA

It’s almost exactly 4 times larger than Earth and has its own set of strange dusty rings – perhaps left over from a shattered moon. It has at least 27 moons, that we know of, and many more interesting features that would fascinate astronomers, if we had a spacecraft there, which we don’t. Which is ridiculous. We’ve only made one close flyby of Uranus by Voyager II back in 1986.

We’ve seen Pluto up close, but there are no plans to visit Uranus? Madness.

Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.
Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.

Anyway, perhaps one of the strangest aspects of Uranus is its tilt. The planet is flipped over on its side, like a Weeble, that wouldn’t unwobble.

Actually, all the planets in the Solar System have some level of axial tilt. The Earth is tilted 23.5 degrees away from the Sun’s equator. Mars is 25 degrees, and even Mercury is 2.1 degrees tilted. These tilts are everywhere.

But Uranus is 97.8 degrees. That’s just 0.2 degrees shy of a 90s boy band.

You might be wondering, why have it be more than 90 degrees. High school geometry tells me that 97.8 degrees is the same as 82.2 degrees. And that’s true. But astronomers define the angle as greater than 90 degrees when you take its direction of rotation into account. When you describe it as turning in the same direction as the rest of the planets in the Solar System, then you have to measure it this way.

What could have done that to Uranus, how could it have happened?

The fact that Uranus is flipped over on its side tells us that the calm clockwork motion of the Solar System hasn’t always been this way. Shortly after the formation of the Sun and planets, our neighborhood was a violent place.

The early planets smashed into each other, pushed one another into new orbits. Some planets could have been spun out of the Solar System entirely, while others might have been driven into the Sun. Our own Moon was likely formed when a Mars-sized object crashed into the Earth. Other moons might have been captured from three body interactions between worlds. It was mayhem.

The Solar System that you see today contains the survivors. Everything that wasn’t delivered a death blow.

And something really tried to deliver a death blow to Uranus, very early after it formed. We know this because the moons of Uranus orbit at the same tilt as the planet’s axis. This means that something smashed into Uranus while it was still surrounded by the disk of gas and dust that its moons formed from.

When the massive collision happened, the planet flipped over, wrenching this disk with it. The moons formed within this new configuration.

Astronomers think it was more complicated than that, however. If it was a single, massive collision, models suggest the planet would just flip over entirely, and end up rotating backwards from the other planets in the Solar System.

It’s more likely that another collision or even a series of collisions put the brakes on Uranus’ end over end roll, putting it into its current configuration. It boggles the mind to think about what must have happened.

Uranus' tilt drastically affects the amount of sunlight the hemispheres receive during its orbit. Credit: NASA, ESA, and A. Feild (STScI)
Uranus’ tilt drastically affects the amount of sunlight the hemispheres receive during its orbit. Credit: NASA, ESA, and A. Feild (STScI)

Having such a huge axial tilt makes a big different to Uranus. As it travels around the Sun in its 84-year orbit, the planet still has its poles pointed at fixed locations in space. This means that it spends 42 years with its northern hemisphere roughly pointed towards the Sun, and 42 years with its southern hemisphere in sunlight.

If you could stand on the north pole of Uranus, the Sun would be directly overhead in the middle of summer, and then it would make bigger and bigger circles until it dipped below the horizon a few decades later. Then you wouldn’t see it for a few decades until it finally reappeared again. It would be very very strange.

Of course, it’s a gas planet, so you can’t stand on it. If you could stand on it, we’d all be marveling at your ability to stand on planets.

Here we are in our calm, ordered Solar System, everything’s business as usual. But if you look around, you realize it’s pretty amazing that our planet is even here. Poor sideways Uranus is a testament to our good luck.

How Far is Mars from the Sun?

With the Scientific Revolution, astronomers became aware of the fact that the Earth and the other planets orbit the Sun. And thanks to Copernicus, Galileo, Kepler, and Newton, the study of their orbits was refined to the point of mathematical precision. And with the subsequent discoveries of Uranus, Neptune, Pluto and the Kuiper Belt Objects, we have come to understand just how varied the orbits of the Solar Planets are.

Consider Mars, Earth’s second-closest neighbor, and a planet that is often referred to as “Earth’s Twin”. While it has many things in common with Earth, one area in which they differ greatly is in terms of their orbits. In addition to being farther from the Sun, Mars also has a much more elliptical orbit, which results in some rather interesting variations in temperature and weather patterns.

Perihelion and Aphelion:

Mars orbits the Sun at an average distance (semi-major axis) of 228 million km (141.67 million mi), or 1.524 astronomical units (over one and a half times the distance between Earth and the Sun). However, Mars also has the second most eccentric orbit of all the planets in the Solar System (0.0934), which makes it a distant second to crazy Mercury (at 0.20563).

This means that Mars’ distance from the Sun varies between perihelion (its closest point) and aphelion (its farthest point). In short, the distance between Mars and the Sun ranges during the course of a Martian year from 206,700,000 km (128.437 million mi) at perihelion and 249,200,000 km (154.8457 million mi) at aphelion – or 1.38 AU and 1.666 AU.

Speaking of a Martian year, with an average orbital speed of 24 km/s, Mars takes the equivalent of 687 Earth days to complete a single orbit around the Sun. This means that a year on Mars is equivalent to 1.88 Earth years. Adjusted for Martian days (aka. sols) – which last 24 hours, 39 minutes, and 35 seconds – that works out to a year being 668.5991 sols long (still almost twice as long).

Mars in also the midst of a long-term increase in eccentricity. Roughly 19,000 years ago, it reached a minimum of 0.079, and will peak again at an eccentricity of 0.105 (with a perihelion distance of 1.3621 AU) in about 24,000 years. In addition, the orbit was nearly circular about 1.35 million years ago, and will be again one million years from now.

Axial Tilt:

Much like Earth, Mars also has a significantly tilted axis. In fact, with an inclination of 25.19° to its orbital plane, it is very close to Earth’s own tilt of 23.439°. This means that like Earth, Mars also experiences seasonal variations in terms of temperature.  On average, the surface temperature of Mars is much colder than what we experience here on Earth, but the variation is largely the same.

. Credit and copyright: Encyclopedia Britannica
Mars eccentric orbit and axial tilt result in considerable seasonal variations. Credit and Copyright: Encyclopedia Britannica

All told, the average surface temperature on Mars is -46 °C (-51 °F). This ranges from a low of -143 °C (-225.4 °F), which takes place during winter at the poles; and a high of 35 °C (95 °F), which occurs during summer and midday at the equator. This means that at certain times of the year, Mars is actually warmer than certain parts of Earth.

Orbit and Seasonal Changes:

Mars’ variations in temperature and its seasonal changes are also related to changes in the planet’s orbit. Essentially, Mars’ eccentric orbit means that it travels more slowly around the Sun when it is further from it, and more quickly when it is closer (as stated in Kepler’s Three Laws of Planetary Motion).

Mars’ aphelion coincides with Spring in its northern hemisphere, which makes it the longest season on the planet – lasting roughly 7 Earth months. Summer is second longest, lasting six months, while Fall and Winter last 5.3 and just over 4 months, respectively. In the south, the length of the seasons is only slightly different.

Mars is near perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winter in the southern hemisphere and summer in the north. As a result, the seasons in the southern hemisphere are more extreme and the seasons in the northern are milder. The summer temperatures in the south can be up to 30 K (30 °C; 54 °F) warmer than the equivalent summer temperatures in the north.

Mars' south polar ice cap, seen in April 2000 by Mars Odyssey. NASA/JPL/MSSS
Mars’ south polar ice cap, seen in April 2000 by the Mars Odyssey probe. Credit: NASA/JPL/MSSS

It also snows on Mars. In 2008, NASA’s Phoenix Lander found water ice in the polar regions of the planet. This was an expected finding, but scientists were not prepared to observe snow falling from clouds. The snow, combined with soil chemistry experiments, led scientists to believe that the landing site had a wetter and warmer climate in the past.

And then in 2012, data obtained by the Mars Reconnaissance Orbiter revealed that carbon-dioxide snowfalls occur in the southern polar region of Mars. For decades, scientists have known that carbon-dioxide ice is a permanent part of Mars’ seasonal cycle and exists in the southern polar caps. But this was the first time that such a phenomena was detected, and it remains the only known example of carbon-dioxide snow falling anywhere in our solar system.

In addition, recent surveys conducted by the Mars Reconnaissance Orbiter, the Mars Science Laboratory, the Mars Orbiter Mission (MOM), the Mars Atmosphere and Volatile Evolution (MAVEN) and the Opportunity and Curiosity Rovers have revealed some startling things about Mars’ deep past.

For starters, soil samples and orbital observation have demonstrated conclusively that roughly 3.7 billion years ago, the planet had more water on its surface than is currently in the Atlantic Ocean. Similarly, atmospheric studies conducted on the surface and from space have proven that Mars also had a viable atmosphere at that time, one which was slowly stripped away by solar wind.

Scientists were able to gauge the rate of water loss on Mars by measuring the ratio of water and HDO from today and 4.3 billion years ago. Credit: Kevin Gill
Scientists were able to gauge the rate of water loss on Mars by measuring the ratio of water and HDO from today and 4.3 billion years ago. Credit: Kevin Gill

Weather Patterns:

These seasonal variations allow Mars to experience some extremes in weather. Most notably, Mars has the largest dust storms in the Solar System. These can vary from a storm over a small area to gigantic storms (thousands of km in diameter) that cover the entire planet and obscure the surface from view. They tend to occur when Mars is closest to the Sun, and have been shown to increase the global temperature.

The first mission to notice this was the Mariner 9 orbiter, which was the first spacecraft to orbit Mars in 1971, it sent pictures back to Earth of a world consumed in haze. The entire planet was covered by a dust storm so massive that only Olympus Mons, the giant Martian volcano that measures 24 km high, could be seen above the clouds. This storm lasted for a full month, and delayed Mariner 9‘s attempts to photograph the planet in detail.

And then on June 9th, 2001, the Hubble Space Telescope spotted a dust storm in the Hellas Basin on Mars. By July, the storm had died down, but then grew again to become the largest storm in 25 years. So big was the storm that amateur astronomers using small telescopes were able to see it from Earth. And the cloud raised the temperature of the frigid Martian atmosphere by a stunning 30° Celsius.

These storms tend to occur when Mars is closest to the Sun, and are the result of temperatures rising and triggering changes in the air and soil. As the soil dries, it becomes more easily picked up by air currents, which are caused by pressure changes due to increased heat. The dust storms cause temperatures to rise even further, leading to Mars’ experiencing its own greenhouse effect.

We have written many interesting articles about the distance of the planets from the Sun here at Universe Today. Here’s How Far Are the Planets from the Sun?, How Far is Mercury from the Sun?, How Far is Venus from the Sun?, How Far is the Earth from the Sun?, How Far is the Moon from the Sun?, How Far is Jupiter from the Sun?, How Far is Saturn from the Sun?, What is Uranus’ Distance from the Sun?, What is the Distance of Neptune from the Sun? and How Far is Pluto from the Sun?

For more information, Astronomy for beginners teaches you how to calculate the distance to Mars.

Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.

How Long Does it Take Mars to Orbit the Sun?

Given it’s similarities to Earth, Mars is often referred to as “Earth’s Twin”. Like Earth, Mars is a terrestrial planet, which means it is composed largely of silicate rock and minerals that are differentiated into a core, mantle and crust. It is also located within the Sun’s “Goldilocks Zone” (aka. habitable zone), has polar ice caps, and once had flowing water on its surface. But beyond these, Mars and Earth are very different worlds.

In addition to their stark contrasts in temperature, surface conditions, and exposure to harmful radiation, Mars also takes a significantly longer time to complete a single orbit of the Sun. In fact, a year on Mars is almost twice as long as a year here on Earth – lasting 686.971 days, which works out to about 1.88 Earth years. And in the course of that orbit, the planet undergoes some rather interesting changes.

Continue reading “How Long Does it Take Mars to Orbit the Sun?”

What is the Average Surface Temperature of the Planets in our Solar System?

It’s is no secret that Earth is the only inhabited planet in our Solar System. All the planets besides Earth lack a breathable atmosphere for terrestrial beings, but also, many of them are too hot or too cold to sustain life. A “habitable zone” which exists within every system of planets orbiting a star. Those planets that are too close to their sun are molten and toxic, while those that are too far outside it are icy and frozen.

But at the same time, forces other than position relative to our Sun can affect surface temperatures. For example, some planets are tidally locked, which means that they have one of their sides constantly facing towards the Sun. Others are warmed by internal geological forces and achieve some warmth that does not depend on exposure to the Sun’s rays. So just how hot and cold are the worlds in our Solar System? What exactly are the surface temperatures on these rocky worlds and gas giants that make them inhospitable to life as we know it?

Mercury:

Of our eight planets, Mercury is closest to the Sun. As such, one would expect it to experience the hottest temperatures in our Solar System. However, since Mercury also has no atmosphere and it also spins very slowly compared to the other planets, the surface temperature varies quite widely.

What this means is that the side exposed to the Sun remains exposed for some time, allowing surface temperatures to reach up to a molten 465 °C. Meanwhile, on the dark side, temperatures can drop off to a frigid -184°C. Hence, Mercury varies between extreme heat and extreme cold and is not the hottest planet in our Solar System.

Venus imaged by Magellan Image Credit: NASA/JPL
Venus is an incredibly hot and hostile world, due to a combination of its thick atmosphere and proximity to the Sun. Image Credit: NASA/JPL

Venus:

That honor goes to Venus, the second closest planet to the Sun which also has the highest average surface temperatures – reaching up to 460 °C on a regular basis. This is due in part to Venus’ proximity to the Sun, being just on the inner edge of the habitability zone, but also to Venus’ thick atmosphere, which is composed of heavy clouds of carbon dioxide and sulfur dioxide.

These gases create a strong greenhouse effect which traps a significant portion of the Sun’s heat in the atmosphere and turns the planet surface into a barren, molten landscape. The surface is also marked by extensive volcanoes and lava flows, and rained on by clouds of sulfuric acid. Not a hospitable place by any measure!

Earth:

Earth is the third planet from the Sun, and so far is the only planet that we know of that is capable of supporting life. The average surface temperature here is about 14 °C, but it varies due to a number of factors. For one, our world’s axis is tilted, which means that one hemisphere is slanted towards the Sun during certain times of the year while the other is slanted away.

This not only causes seasonal changes, but ensures that places located closer to the equator are hotter, while those located at the poles are colder. It’s little wonder then why the hottest temperature ever recorded on Earth was in the deserts of Iran (70.7 °C) while the lowest was recorded in Antarctica (-89.2 °C).

Mars' thin atmosphere, visible on the horizon, is too weak to retain heat. Credit: NASA
Mars’ thin atmosphere, visible on the horizon, is too weak to retain heat. Credit: NASA

Mars:

Mars’ average surface temperature is -55 °C, but the Red Planet also experiences some variability, with temperatures ranging as high as 20 °C at the equator during midday, to as low as -153 °C at the poles. On average though, it is much colder than Earth, being just on the outer edge of the habitable zone, and because of its thin atmosphere – which is not sufficient to retain heat.

In addition, its surface temperature can vary by as much as 20 °C due to Mars’ eccentric orbit around the Sun (meaning that it is closer to the Sun at certain points in its orbit than at others).

Jupiter:

Since Jupiter is a gas giant, it has no solid surface, so it has no surface temperature. But measurements taken from the top of Jupiter’s clouds indicate a temperature of approximately -145°C. Closer to the center, the planet’s temperature increases due to atmospheric pressure.

At the point where atmospheric pressure is ten times what it is on Earth, the temperature reaches 21°C, what we Earthlings consider a comfortable “room temperature”. At the core of the planet, the temperature is much higher, reaching as much as 35,700°C – hotter than even the surface of the Sun.

Saturn and its rings, as seen from above the planet by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute. Assembled by Gordan Ugarkovic.
Saturn and its rings, as seen from above the planet by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute/Gordan Ugarkovic

Saturn:

Due to its distance from the Sun, Saturn is a rather cold gas giant planet, with an average temperature of -178 °Celsius. But because of Saturn’s tilt, the southern and northern hemispheres are heated differently, causing seasonal temperature variation.

And much like Jupiter, the temperature in the upper atmosphere of Saturn is cold, but increases closer to the center of the planet. At the core of the planet, temperatures are believed to reach as high as 11,700 °C.

Uranus:

Uranus is the coldest planet in our Solar System, with a lowest recorded temperature of -224°C. Despite its distance from the Sun, the largest contributing factor to its frigid nature has to do with its core.

Much like the other gas giants in our Solar System, the core of Uranus gives off far more heat than is absorbed from the Sun. However, with a core temperature of approximately 4,737 °C, Uranus’ interior gives of only one-fifth the heat that Jupiter’s does and less than half that of Saturn.

Neptune photographed by Voyage. Image credit: NASA/JPL
Neptune photographed by Voyager 2. Image credit: NASA/JPL

Neptune:

With temperatures dropping to -218°C in Neptune’s upper atmosphere, the planet is one of the coldest in our Solar System. And like all of the gas giants, Neptune has a much hotter core, which is around 7,000°C.

In short, the Solar System runs the gambit from extreme cold to extreme hot, with plenty of variance and only a few places that are temperate enough to sustain life. And of all of those, it is only planet Earth that seems to strike the careful balance required to sustain it perpetually.

Universe Today has many articles on the temperature of each planet, including the temperature of Mars and the temperature of Earth.

You may also want to check out these articles on facts about the planets and an overview of the planets.

NASA has a great graphic here that compares the temperatures of all the planets in our Solar System.

Astronomy Cast has episodes on all planets including Mercury.

The Orbit of Earth. How Long is a Year on Earth?

Ever since the 16th century when Nicolaus Copernicus demonstrated that the Earth revolved around in the Sun, scientists have worked tirelessly to understand the relationship in mathematical terms. If this bright celestial body – upon which depends the seasons, the diurnal cycle, and all life on Earth – does not revolve around us, then what exactly is the nature of our orbit around it?

For several centuries, astronomers have applied the scientific method to answer this question, and have determined that the Earth’s orbit around the Sun has many fascinating characteristics. And what they have found has helped us to understanding why we measure time the way we do.

Orbital Characteristics:

First of all, the speed of the Earth’s orbit around the Sun is 108,000 km/h, which means that our planet travels 940 million km during a single orbit. The Earth completes one orbit every 365.242199 mean solar days, a fact which goes a long way towards explaining why need an extra calendar day every four years (aka. during a leap year).

The planet’s distance from the Sun varies as it orbits. In fact, the Earth is never the same distance from the Sun from day to day. When the Earth is closest to the Sun, it is said to be at perihelion. This occurs around January 3rd each year, when the Earth is at a distance of about 147,098,074 km.

The average distance of the Earth from the Sun is about 149.6 million km, which is also referred to as one astronomical unit (AU). When it is at its farthest distance from the Sun, Earth is said to be at aphelion – which happens around July 4th where the Earth reaches a distance of about 152,097,701 km.

And those of you in the northern hemisphere will notice that “warm” or “cold” weather does not coincide with how close the Earth is to the Sun. That is determined by axial tilt (see below).

Elliptical Orbit:

Next, there is the nature of the Earth’s orbit. Rather than being a perfect circle, the Earth moves around the Sun in an extended circular or oval pattern. This is what is known as an “elliptical” orbit. This orbital pattern was first described by Johannes Kepler, a German mathematician and astronomer, in his seminal work Astronomia nova (New Astronomy).

An illustration of Kepler's three laws of motion, which show two planets that have elliptical orbits around the Sun. Credit: Wikipedia/Hankwang
An illustration of Kepler’s three laws of motion, which show two planets that have elliptical orbits around the Sun. Credit: Wikipedia/Hankwang

After measuring the orbits of the Earth and Mars, he noticed that at times, the orbits of both planets appeared to be speeding up or slowing down. This coincided directly with the planets’ aphelion and perihelion, meaning that the planets’ distance from the Sun bore a direct relationship to the speed of their orbits. It also meant that both Earth and Mars did not orbit the Sun in perfectly circular patterns.

In describing the nature of elliptical orbits, scientists use a factor known as “eccentricity”, which is expressed in the form of a number between zero and one. If a planet’s eccentricity is close to zero, then the ellipse is nearly a circle. If it is close to one, the ellipse is long and slender.

Earth’s orbit has an eccentricity of less than 0.02, which means that it is very close to being circular. That is why the difference between the Earth’s distance from the Sun at perihelion and aphelion is very little – less than 5 million km.

Seasonal Change:

Third, there is the role Earth’s orbit plays in the seasons, which we referred to above. The four seasons are determined by the fact that the Earth is tilted 23.4° on its vertical axis, which is referred to as “axial tilt.” This quirk in our orbit determines the solstices – the point in the orbit of maximum axial tilt toward or away from the Sun – and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular.

Over the course of a year the orientation of the axis remains fixed in space, producing changes in the distribution of solar radiation. These changes in the pattern of radiation reaching earth’s surface cause the succession of the seasons. Credit: NOAA/Thomas G. Andrews
Over the course of a year the orientation of the axis remains fixed in space, producing changes in the distribution of solar radiation. Credit: NOAA/Thomas G. Andrews

In short, when the northern hemisphere is tilted away from the Sun, it experiences winter while the southern hemisphere experiences summer. Six months later, when the northern hemisphere is tilted towards the Sun, the seasonal order is reversed.

In the northern hemisphere, winter solstice occurs around December 21st, summer solstice is near June 21st, spring equinox is around March 20th and autumnal equinox is about September 23rd. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

While it is true that Earth does have a perihelion, or point at which it is closest to the sun, and an aphelion, its farthest point from the Sun, the difference between these distances is too minimal to have any significant impact on the Earth’s seasons and climate.

Lagrange Points:

Another interesting characteristic of the Earth’s orbit around the Sun has to do with Lagrange Points. These are the five positions in Earth’s orbital configuration around the Sun where where the combined gravitational pull of the Earth and the Sun provides precisely the centripetal force required to orbit with them.

Sun Earth Lagrange Points. Credit: Xander89/Wikimedia Commons
Sun-Earth Lagrange Points. Credit: Xander89/Wikimedia Commons

The five Lagrange Points between the Earth are labelled (somewhat unimaginatively) L1 to L5. L1, L2, and L3 sit along a straight line that goes through the Earth and Sun. L1 sits between them, L3 is on the opposite side of the Sun from the Earth, and L2 is on the opposite side of the Earth from L1. These three Lagrange points are unstable,  which means that a satellite placed at any one of them will move off course if disturbed in the slightest.

The L4 and L5 points lie at the tips of the two equilateral triangles where the Sun and Earth constitute the two lower points. These points liem along along Earth’s orbit, with L4 60° behind it and L5 60° ahead.  These two Lagrange Points are stable, hence why they are popular destinations for satellites and space telescopes.

The study of Earth’s orbit around the Sun has taught scientists much about other planets as well. Knowing where a planet sits in relation to its parent star, its orbital period, its axial tilt, and a host of other factors are all central to determining whether or not life may exist on one, and whether or not human beings could one day live there.

We have written many interesting articles about the Earth’s orbit here at Universe Today. Here’s 10 Interesting Facts About Earth, How Far is Earth from the Sun?, What is the Rotation of the Earth?, Why are there Seasons?, and What is Earth’s Axial Tilt?

For more information, check out this article on NASA- Window’s to the Universe article on elliptical orbits or check out NASA’s Earth: Overview.

Astronomy Cast also espidoes that are relevant to the subject. Here’s BQuestions Show: Black black holes, Unbalancing the Earth, and Space Pollution.

Sources:

Milankovitch Cycle

Milankovitch cycles. Source: UCAR

A Milankovitch cycle is a cyclical movement related to the Earth’s orbit around the Sun. There are three of them: eccentricity, axial tilt, and precession. According to the Milankovitch Theory, these three cycles combine to affect the amount of solar heat that’s incident on the Earth’s surface and subsequently influence climatic patterns.

Eccentricity

The path of the Earth’s orbit around the sun is not a perfect circle, but an ellipse. This elliptical shape changes from less elliptical (nearly a perfect circle) to more elliptical and back, and is due to the gravitational fields of neighboring planets (particularly the large ones – Jupiter and Saturn). The measure of the shape’s deviation from being a circle is called its eccentricity.

That is, the larger the eccentricity, the greater is its deviation from a circle. Thus, in terms of eccentricity, the Earth’s orbit undergoes a cyclical change from less eccentric to more eccentric and back. One complete cycle for this kind of variation lasts for about 100,000 years.

Axial Tilt

We know the earth is spinning around its own axis, which is the reason why we have night and day. However, this axis is not upright. Rather, it tilts at angles between 22.1-degrees and 24.5 degrees and back. These angles are measured between the angle of the axis to an imaginary line normal (perpendicular) to the Earth’s plane of orbit. A complete cycle for the axial tilt lasts for about 41,000 years.

Greater tilts mean that the hemispheres closer to the Sun, i.e., during summer, will experience a larger amount of heat than when the tilt is less. In other words, regions in the extreme upper and lower hemispheres will experience the hottest summers and the coldest winters during a maximum tilt.

Precession

Aside from the tilt, the axis also wobbles like a top. A complete wobble cycle is more or less 26,000 years. This motion is caused by tidal forces from the Sun and Moon.

Precession as well as tilting are the reasons why regions near and at the poles experience very long nights and very long days at certain times of the year. For example, in Norway, the Sun never completely descends beneath the horizon between late May to late July.

The Milankovitch Cycles are among the arguments fielded by detractors of the Global Warming concept. According to them, the Earth’s current warming is just a part of a series of cyclical events that take thousands of years to complete, and hence cannot be prevented.

You can read more about milankovitch cycle here in Universe Today. Here are the links:

There’s more about it at USGS and NASA. Here are a couple of sources there:

Here are two episodes at Astronomy Cast that you might want to check out as well:

References:
NASA Earth Observatory
NOAA Website